The present invention relates to the stabilization of surfaces of a compound semiconductor to be used for a semiconductor device and, in particular, to a fabrication technique appropriate to prevent the deterioration of the light-emitting end surface of a semiconductor laser device.
In recent years, semiconductor laser devices have been broadly used as key components for optical disks and optical communications.
In a typical conventional semiconductor laser device, a reflecting film (typically, a dielectric film) for regulating the reflectance is formed at a light-emitting portion, and this film has been used for protecting the light-emitting portion and controlling the reflectance. For this film, Al2O3 (alumina), SiOx (silicon oxide), and SiNx (silicon nitride) have been typically used. Also, a process such as electron beam evaporation (EB evaporation), plasma CVD, ECR-CVD, or sputtering has been typically used to form the reflecting film.
A semiconductor interface state is generated at the light-emitting end surface formed by the aforementioned conventional process. In addition to this, a very intense light passes through this end surface. This has led to the problem that the light-emitting end surface tends to deteriorate, particularly during a high-power operation.
As a solution to this problem, there has been a method in which a bar obtained by cleaving a semiconductor laser device wafer is immersed in a sulfur-containing solution (ammonium sulfide) to thereby form a film containing sulfur of several atomic layers on a cavity end surface, and further a protective layer of Si3N4 or the like is formed on the film, as taught in, for example, the Japanese Patent Laid-Open Publication No. HEI 3-149889.
However, the conventional method of forming the end surface protective layer has the following problems.
That is, if the sulfur atoms adhere to an AlGaAs semiconductor surface, then the interface states are restrained, whereby photoabsorption is inhibited. However, the sulfur layer formed by the solution treatment exhibits weak bond to AlGaAs, and this disadvantageously leads to the detachment of the greater part of the sulfur layer from AlGaAs during the protective layer depositing process.
When forming the protective layer by an electron beam evaporation method on the semiconductor laser device light-emitting end surface that has undergone sulfur treatment, the electron beam impinges on the source of evaporation to heat it up to an elevated temperature to thereby carry out the evaporation. In this stage, the ionized molecules for deposit, part of the electron beam, intense light and so on reach the semiconductor laser device end surface, and these matters act to remove the sulfur layer from the AlGaAs surface.
Furthermore, in case that a compact dielectric protective layer having good adhesion is formed, the electron beam is intensified, and this has caused the problem that the effect of the sulfur treatment is disadvantageously reduced by a large quantity.
When forming a protective layer by a method using plasma, more specifically, an ECR-CVD method, a plasma CVD method, or a sputtering method, instead of the electron beam evaporation method, the plasma impinges on the sulfur layer, removing the sulfur layer from the AlGaAs surface. Therefore, this technique also has the problem that the effect of the sulfur treatment is disadvantageously reduced by a large quantity.
Besides the above method of forming a sulfur layer, a method as a second prior art technique is proposed by, for example, the Japanese Patent Laid-Open Publication No. HEI 7-176819 in which a bar obtained by cleaving a semiconductor laser device wafer is immersed in an ammonium sulfide solution while being irradiated with light, to thereby form a polymolecular layer of sulfur on the semiconductor laser light-emitting end surface.
The polymolecular layer of sulfur formed in this method serves to prevent the sulfur layer from coming off even when ultraviolet rays are applied in the subsequent process.
However, the process of applying light to the bar in the solution has had a problem that, due to difficulties in uniformly dispersing light and the fact that the sulfur polymolecular layer tends to volatilize on an elevated temperature condition (the melting point of monoclinic sulfur is 119xc2x0 C.) similarly to the aforementioned sulfur layer, the polymolecular layer does not provide sufficient protection for the end surface in depositing a reflecting film at an elevated substrate temperature.
As a third prior art technique, for example, the Japanese Patent Laid-Open Publication No. HEI 4-345079 proposes a method in which after the semiconductor laser device light-emitting end surface has been subjected to an ammonium sulfide solution treatment, a II-VI semiconductor single crystal (such as ZnS) is formed in a high vacuum by an MBE (molecular-beam epitaxy) method.
However, this method requires use of an expensive MBE apparatus. Furthermore, the method using the MBE apparatus has a problem that a technique for crystal growth, which is very hard to control, is needed.
The crystal growth by the MBE method is generally carried out after the formation of an electrode, and this has caused a problem that the satisfactory growth of the II-VI semiconductor single crystal is hard to achieve due to contamination by the electrode or substances adhering to the electrode.
There is a further problem as follows. It is difficult to grow a uniform single crystal II-VI semiconductor on the cleaved surface of AlGaAs, so that unevenness called the hillock frequently occurs. Furthermore, if a single crystal is formed on the semiconductor laser device end surface, there may occur a distortion in the inside of the semiconductor laser device due to differences in coefficient of thermal expansion and lattice constant, eventually causing deterioration of the laser device.
The present invention has been made to solve the aforementioned problems and has an object to stabilize a surface of a compound semiconductor, such as light-emitting end surfaces of a semiconductor laser device, so that sulfur provided on the surface of the compound semiconductor surface is not detached by the influence of evaporation and the like.
The present invention has another object to increase the lifetime of the semiconductor laser device particularly during a high-power operation.
The above objects are achieved by a compound semiconductor surface stabilizing method comprising steps of:
immersing a region that includes a surface of a compound semiconductor in a solution containing sulfur ions; and
immersing the region that includes the surface of the compound semiconductor in a solution containing cations which react with sulfur to form a sulfide.
Note that the xe2x80x9cregion that includes a surfacexe2x80x9d of a compound semiconductor may be part of or whole the compound semiconductor.
According to the above arrangement, the sulfur layer is formed by immersing the region that includes a surface of the compound semiconductor in the solution containing sulfur ions, and thereafter the sulfide layer for protecting the sulfur layer is formed by immersing the sulfur layer in the cationic solution that generates a sulfide through reaction with sulfur. The sulfide layer prevents the sulfur layer from being detached from the surface by application of heating, electrons, ions inside plasma, and light or the like. Therefore, the problem of the coming-off, or detachment, of sulfur that has been caused by the prior art technique (as disclosed in, for examples Japanese Patent Laid-Open Publication No. HEI 3-149889) can be solved. Furthermore, the sulfide layer formed in accordance with the invention is superior in stability at high temperatures to the sulfur polymolecular layer formed by the second prior art technique (as disclosed in, for example, Japanese Patent Laid-Open Publication No. HEI 7-176819), and this can prevent the detachment of sulfur even if a high-temperature treatment is performed. Taking advantage of the solution reaction to expose the remaining sulfur ions to the cationic solution, an amorphous or polycrystalline sulfur layer and an amorphous or polycrystalline sulfide layer are formed. Therefore, the sulfide layer can be formed simply and readily, as compared with the third prior art technique (as disclosed in, for example, Japanese Patent Laid-Open Publication No. HEI 4-345079), and yet the formed sulfide gives no strain to the inside of the compound semiconductor, unlike the case of the single crystal. According to the third prior art technique, the sulfur layer that has been once provided through the solution treatment is made to come off, except for only a monomolecular layer, by a heat treatment at a high temperature of 350xc2x0 C. whereby a II-VI semiconductor single crystal is formed on the remaining monomolecular layer. In contrast to this, according to the present invention, the sulfur layer is prevented from coming off in the sulfide layer forming stage because the sulfide layer is formed without carrying out such a high-temperature treatment.
In one embodiment, between the step of immersing a region that includes a surface of a compound semiconductor in a solution containing sulfur ions and the step of immersing the region that includes the surface of the compound semiconductor in a solution containing cations which react with sulfur to form a sulfide, the method further comprises a step of rinsing the surface of the compound semiconductor.
The method may further include a step of rinsing the surface of the compound semiconductor after the step of immersing the region that includes the surface of the compound semiconductor in a solution containing cations which react with sulfur to form a sulfide.
With such an arrangement, the sulfur layer is formed on the surface by immersing the compound semiconductor in the sulfur solution, and thereafter excessive sulfur is removed by rinsing, or washing in water. Subsequently, the resulting semiconductor is immersed in the cationic solution that causes a sulfide, and the rinsing may further performed as necessary. As a result, a thin uniform sulfide protective layer is formed on the compound semiconductor surface. With this method, nonuniform deposition of sulfide, which might occur in the case where rinsing is not performed, is prevented.
By repeating the above-mentioned four process steps (i.e., the steps of immersing the compound semiconductor in the sulfur solution to form the sulfur layer on its surface, removing the excessive sulfur by performing rinsing, thereafter immersing the resulting semiconductor in the cationic solution that causes a sulfide, and performing rinsing), a uniform thick sulfide protective layer can be formed on the compound semiconductor surface. Thus, the sulfide becomes more operative as the protective layer.
As the solution containing sulfur ions, any of an ammonium sulfide solution (colorless), an ammonium sulfide solution (yellow), a sodium sulfide solution, and a potassium sulfide solution can be used.
The ammonium sulfide solution (colorless), ammonium sulfide solution (yellow) (also called an ammonium polysulfide solution), sodium sulfide solution and potassium sulfide solution have high rates of ionization to sulfur ions. Therefore, if any one of these solutions is used, a sulfur layer can be effectively formed on the surface of the compound semiconductor.
A solution containing cations of any of Zn, Cd and Ca may be used as the solution containing cations.
The solution containing Zn, Cd or Ca, which has a low solubility to water and in which a stable sulfide is formed, is suitable for the solution treatment. Furthermore, because the sulfide formed is a stable compound having a high sublimation temperature or melting point, it is possible to perform a process using a high temperature after the solution treatment. For example, even when a film of alumina or the like is formed at a high temperature on the sulfide, the latter does not come off, which means that a groundwork or liner protecting effect of the sulfide is retained.
If the compound semiconductor contains any of elements of As, P, Al, Ga and In, the solution containing cations is preferably a solution containing at least cations of As, P, Al, Ga or In that is contained in the compound semiconductor.
According to the above arrangement, by using the solution containing the constituent element of As, P, Al, Ga or In of the compound semiconductor as the cationic solution, little influence is exerted on the matrix or base of the compound semiconductor even when elements diffuse from the solution into the compound semiconductor, and vice versa, during a high-temperature treatment. Furthermore, because a stable compound having a high sublimation temperature or melting point is formed, it is possible to incorporate a process using a high temperature after the solution treatment. For example, even when a film of alumina or the like is formed at a high temperature on the sulfide, the latter does not come off, which means that a groundwork protecting effect is continuously retained.
In one embodiment, the solution containing cations is a solution containing both cations and acetate ions.
With this arrangement, taking advantage of the fact that acetic acid is a weak acid, a moderate protective layer formation can be achieved without etching the compound surface.
In one embodiment, the solution containing cations is a zinc acetate solution.
With the zinc acetate solution being used as the cationic solution, a protective zinc sulfide layer, which has a low water-solubility and does not absorb light at the wavelength of emissions from the semiconductor laser device, can be formed through a moderate process that performs no etching of surfaces of the compound.
The solution containing cations may be either a solution containing both cations and chlorine ions or a solution containing both cations and sulfate ions.
In this case, taking advantage of the fact that hydrochloric acid (chlorine ions) and sulfuric acid are strong acids, oxygen and the like is removed from the compound surface. This improves the effect of the sulfur treatment and further allows the formation of the protective layer with sulfide.
In one embodiment, the solution containing cations is a zinc chloride solution or a zinc sulfate solution.
According to the above arrangement, taking advantage of the fact that hydrochloric acid and sulfuric acid are strong acids, the removal of oxygen and the like on the compound surface can be achieved. Also, it is possible to form a zinc sulfide protective layer that has a low water-solubility and does not absorb light at the wavelength of emissions from the semiconductor laser device.
The above object is also achieved by a semiconductor device, according to another aspect of the invention, comprising a compound semiconductor, and an amorphous or polycrystalline sulfur layer and an amorphous or polycrystalline sulfide layer formed in this order on a surface of the compound semiconductor.
In one embodiment, the semiconductor device is a semiconductor laser device, and the surface of the compound semiconductor is an end surface that includes a light-emitting portion.
According to this arrangement, because each of the sulfur layer and the sulfide layer is not monocrystalline, but amorphous or polycrystalline, no distortion is generated inside the semiconductor device. Therefore, the device deterioration attributed to the internal distortion is prevented. Accordingly, the life of the device can be improved.
The sulfide layer may preferably have a thickness of 350 xc3x85 or less. With this film thickness, satisfactory device characteristics can be obtained because the device suffers almost no influence of the slope efficiency deterioration which will otherwise take place due to the light absorption and dispersion in the sulfide layer.
The semiconductor laser device with the above construction can be fabricated by sequentially forming a sulfur layer and a sulfide layer on an end surface including a light-emitting portion of the semiconductor laser device by the compound semiconductor surface stabilizing method according to the present invention.
According to the present invention, the region that includes the semiconductor laser light-emitting end surface is sequentially immersed in the solution containing sulfur ions and the solution containing cations that generate a sulfide through reaction with sulfur. As a result, the sulfur layer and the sulfide layer are formed in this order on the end surface. Thus, the sulfur layer, which serves to restrain the device deterioration attributed to the end surface deterioration, is covered with the sulfide layer. Therefore, the sulfur layer keeps stable even in a long-term continuous operation. Accordingly, reliability of the device is improved as compared with the prior art technique of providing only the sulfur layer (for example, Japanese Patent Laid-Open Publication No. HEI 3-149889). Furthermore, the sulfur layer can be covered with a stable sulfide having a sublimation temperature higher than that of the sulfur polymolecular layer that has been formed by the second prior art technique (for example, Japanese Patent Laid-Open Publication No. HEI 7-176819). In addition, unlike the formation of the polymolecular layer by the second prior art technique, formation of the sulfide layer does not require irradiation with light, which allows the semiconductor laser to be fabricated through simpler processes. Dissimilar to the third prior art technique (for example, Japanese Patent Laid-Open Publication No. HEI 4-345079), the sulfide layer can be simply fabricated continuously to the solution treatment for forming the sulfur layer. The sulfide layer formed is not monocrystalline but amorphous or polycrystalline, and therefore, no strain is caused inside the device. Furthermore, according to the third prior art technique, the sulfur layer that has been once provided through the solution treatment is made to come off, except for only a monomolecular layer, by a heat treatment at a high temperature of 350xc2x0 C. whereby a II-VI semiconductor single crystal is formed on the remaining monomolecular layer. In, contrast to this, according to the present invention, the sulfur layer is prevented from coming off in the sulfide layer forming stage because the sulfide layer is formed without carrying out such a high-temperature treatment.
The semiconductor laser device may be provided with a reflecting film on the sulfide layer. According to this arrangement, the sulfide layer functions as a barrier layer between the sulfur layer and the reflecting film. That is, the sulfide layer serves to prevent the sulfur layer from gradually loosing its effect while diffusing into the reflecting film during a long-time operation and causing the deterioration of the semiconductor laser device. According to the third prior art technique, no reflecting film is provided on the ZnS film.
Formation of the reflecting film on the sulfide layer can by performed by electron beam evaporation, plasma CVD, ECR-CVD, sputtering, or any other method using plasma.
According to the present invention, the impact upon the end surface of the electron beam, ion beam or light applied in the electron beam evaporation stage or of the plasma ions in the plasma methods is remarkably restrained or suppressed by the sulfide layer. This arrangement allows the sulfur layer, which is intended to prevent the device deterioration attributed to the deterioration of the end surface, to be stabilized for a long-time continuous use, which eventually leads to improvement of reliability of the device.
Other objects, features and advantages of the present invention will be obvious from the following description.